1. Field of the Invention
The present invention relates to a method for producing a lanthanum chromite film and a method for producing an interconnector using the film for solid electrolyte type fuel cells.
2. Description of the Related Art
Recently, fuel cells have attracted attention as electric power generating apparatuses which can directly convert chemical energy of fuels into electrical energy. They are not restricted by Carnot cycle, so that they have essentially high efficiency in energy conversion and can use a variety of fuels, such as, naphtha, natural gas, methanol, coal-reformed gas, and heavy oil. In addition, they show low environmental pollution and their efficiency of electrical power generation is not influenced by a scale of the generation plant, so that they are now a very promising technique.
Particularly, solid electrolyte type fuel cells (to be referred to as "SOFC" hereinafter) are operated at a high temperature of around 1,000.degree. C. Therefore, reactions at the electrodes proceed virgorously without necessitating an expensive noble metal catalyst, such as platinum, polarization of the electrodes is small, and output voltage of the cell is relatively high, so that they have an exceedingly high efficiency in energy conversion as compared with the other fuel cells. Moreover, SOFC are constructed wholly from solid materials, so that they are stable and exhibit a long life.
In these SOFC, generally a fuel electrode and an air electrode of an adjacent element (unit cell) of SOFC are connected in series via an interconnector and joining terminals. Therefore, a thin interconnector is particularly desired in SOFC so as to decrease the electrical resistance therefor.
As a technique of producing a thin interconnector, a chemical vapor deposition (CVD) method or electrochemical vapor deposition (EVD) method can be considered. However, these methods require a large apparatus for forming the interconnector film and can be performed only on a small deposition area and at a slow speed.
A technique of using a plasma thermal spray has been used in the production of SOFC in that it is quick in film-forming speed, simple, and affords a thin and relatively dense film, as described in "Sunshine" Vol. 12, No. 1, 1981, for example. Also, Japanese patent application laid-open Nos. 61-198,569 and 61-198,570 disclose that a raw material consisting of a solid solution of cerium oxide or zirconium oxide and an oxide of a metal of alkaline earth element or rare earth element is pulverized to a desired fineness, and plasma thermal sprayed to form a solid electrolyte film.
However, a film formed by plasma thermal spray has generally such a large porosity that it is poor in airtight property as an interconnector for SOFC, and has cracks or layered defects therein already at the time of completing the plasma thermal spraying. Therefore, fuel leakage occurs during operation of the SOFC to leak hydrogen, carbon monoxide, etc., through the interconnector. As a result, an electromotive force per unit cell of the SOFC becomes smaller than the usual 1 volt, for example, to reduce output of the SOFC and decrease conversion rate of the fuel into electrical power. In order to solve the disadvantages, a way of increasing the interconnector film thickness to prevent the fuel leakage has been considered. However, in such a method, inherent resistance of the SOFC is increased to decrease the output of the SOFC. Therefore, a way has been earnestly desired which can improve airtight property of the interconnector film while making the film thickness thin, in order to improve the output of the SOFC as far as the fuel leakage is prevented.
Meanwhile, lanthanum chromite generally used as a material for the interconnector has such a characteristic property that it can hardly be densified by sintering thereof, so that a densifying agent, such as copper, or zinc, etc., has to be added thereto for withstanding practical use, otherwise lanthanum chromite having an extremely small relative density is obtained.